This invention relates to a device for a vibration apparatus, in particular a small appartus, such as a dry shaver, which is driven by a single-phase synchronous motor whose shaft carries at least one cam which cooperates resiliently with pressure means.
Such a device is known from U.S. Pat. No. 3,984,710. In this device pressure rollers, which are arranged on slides so as to be movable towards and away from one another, are urged against the cam from opposite directions. This device constitutes a starting aid and has no function in the actual drive mechanism. The spring-loaded pressure rollers ensure that after the stator has been de-energized, even in the case of high friction, the cam rotates the rotor in such a way that starting is always guaranteed or, in other words, that the principal direction of magnetization of the motor is rotated in such a way relative to the principal stator-field direction that the two directions make an angle with one another. If the two directions of magnetization coincide, the motor cannot start.
Further, it is known to drive a vibration apparatus, for example a vibration-type dry-shaver, by means of a rotary motor (see U.S. Pat. No. 4,400,875). For this purpose, two cam discs are mounted on the rotor shaft to drive the cutter of the dry shaver through a hinged lever system. This lever system is vulnerable because the individual parts are linked by integral hinges and, when it is constructed as a crank drive, it produces annoying noises.
From U.S. Pat. No. 4,400,875 it is known to use springs which in combination with the cutter form a resonant system which is tuned by means of the springs. However, these springs do not serve to rotate the rotor out of the parallel position and therefore do not constitute a starting aid in the case of high-friction loads. According to this patent it is required that in the parallel position of the rotor, the cutter is in one of the two positions of maximum excursion. In these positions the springs cannot exert torque on the rotor because the line of action of the spring force extends through the point of contact between the cam and the roller and through the rotor axis. Further, the main purpose of the springs is not to maintain contact between a cam and a roller. This contact is rather maintained by clamping two cams between two rollers.
Moreover, U.S. Pat. No. 4,400,875 also shows a drive system in which a cam drives an oscillating lever. What has been stated above about the springs also applies to this construction.